Fix compile error caused by c14n support on older versions of libxml.
Fix compile error caused by c14n support on older versions of libxml.

sferik/libxml-ruby,increments/libxml-ruby,xml4r/libxml-ruby,increments/libxml-ruby,sferik/libxml-ruby,sferik/libxml-ruby,sferik/libxml-ruby,sferik/libxml-ruby,sferik/libxml-ruby,xml4r/libxml-ruby,xml4r/libxml-ruby,increments/libxml-ruby